1. Field of the Invention
The present invention relates to silver halide photographic emulsions which are spectrally sensitized, particularly, those which are suitable for producing a red-sensitive emulsion layer of a multilayer color light-sensitive material.
2. Description of the Prior Art
Spectral sensitization is an extremely important technique which has been used to not only impart spectral sensitivity to a silver halide photographic emulsion ranging from the sensitization region intrinsic to the silver halide to the longer wavelength region corresponding to green light and red light, but also to increase the total sensitivity thereof (to white light). Particularly, since multilayer color photosensitive materials require respective photographic emulsion layers sensitive to not only blue light but also to green light and to red light, spectral sensitization is indispensable for the production of a green-sensitive emulsion layer and a red-sensitive emulsion layer.
Furthermore, a more suitable color reproducibility for color light-sensitive materials is strongly required. Therefore, the characteristics of spectral sensitivity of a green-sensitive layer and a red-sensitive layer are, from the standpoint of obtaining excellent color reproducibility, significant. In particular, it is important to select the red-sensitive layer because the wavelength of the maximum sensitivity of the red-sensitive layer delicately affects the reproduction of skin color, which is a significant factor in color photographs, and also influences variations of color balance caused by changes in the light source used for exposure, as disclosed in U.S. Pat. No. 3,672,898, corresponding to Japanese Patent Publication No. 6207/74. For example, Japanese Patent Publication No. 6207/74 discloses that the selection of about 605 nm (.+-.5 nm) as the wavelength of the maximum sensitivity of the red-sensitive layer in a multilayer color light-sensitive material is desirable to maintain color balance under the best condition when three kinds of sources of light for exposure are used, i.e., day light, light from a tungsten lamp and light from a fluorescent lamp. However, it is not preferred, for the purpose of producing highly sensitive color light-sensitive materials for camera exposure, to choose the above-described wavelength as that of the maximum sensitivity of the red-sensitive emulsion layer because such an emulsion does not have a high sensitivity, though the smallest variations in color balance are achieved therewith in using the light sources described above, and, therefore, it is necessary for the grain size of silver halide in the photographic emulsion to be large to increase the sensitivity. Unfortunately, an increase in grain size results in a sacrifice of graininess and sharpness.
It was found that if the red-sensitive layer had a maximum sensitivity within the wavelength region ranging from 625 nm to 645 nm and in addition, a spectral sensitivity in the wavelength region ranging from 580 nm to 600 nm corresponding to at least 40% of the maximum spectral sensitivity was most advantageous to obtain highly sensitive color light-sensitive materials which possess a satisfactory high color reproducibility without deteriorating both graininess and sharpness.
When spectral sensitization is carried out with a combination of two or more sensitizing dyes or of a sensitizing dye and an organic heterocyclic compound in order to render the photographic emulsion spectrally sensitive to the above-described wavelength region, sometimes a decrease in spectral sensitivity in a specific spectral sensitized wavelength region, an increase in fog, a deterioration of stability with the lapse of time such as decreasing of sensitivity and increasing of fog during or after production of the light-sensitive materials and a deterioration of the stability of the latent image with the lapse of time of from photographing to development occur. It can thus be seen that many combinations of sensitizing dyes are unsuitable for producing a spectrally sensitized photographic emulsion having desired photographic properties. One of the most important problems in the art of producing light-sensitive materials is to spectrally sensitize by selecting and then combining sensitizing dyes which do not adversely affect the photographic properties thereof.
Moreover, a requirement for a sensitizing dye is for the spectral sensitization action inherent to the sensitizing dye not to be inhibited by couplers which may be co-present therewith, since modern multilayer color light-sensitive materials contain color image-forming couplers in the respective photographic emulsion layers. Furthermore, coloration due to residual dye must not appear after photographic processings, since the hue of the color photograph obtained is remarkably impaired by such coloration.